In recent years, there has been increasing interest in metal halide lamps, i.e., electric discharge devices which emit color-modified light and contain various metal halides as additives. The useful life of these lamps may range from 6,000 to over 10,000 hours, and the efficiency, measured in lumens of light per watt, of metal halide lamps in many instances is greater than 100.
In the preparation of these lamps, additive metal halides have been pulverized or powdered, and then pelleted in a desired mass or weight for machine feeding to the arc tube or envelope of the lamp. These metal halides, however, have contained impurities such as water and various hydroxides. Flaming, i.e., heating with a hand torch, has been used to drive off the water, but this procedure does not normally remove the hydroxide impurities, and has a further disadvantage in that other volatile material, desirably left in the lamp, is also driven off with the water.
The presence of hydrogen and oxygen in any form within the lamp envelope is detrimental to the lamp. These detriments may exist not only as free hydrogen and oxygen, but also as compounds thereof such as water, hydroxides, sulfates and silicates. The oxygen present within the envelope of the lamp oxidizes the metals such as tungsten which make up the lamp electrodes or filaments, and the resulting metal oxide condenses upon the interior surface of the lamp envelope thereby reducing lamp efficiency. The hydrogen present within the envelope of the lamp then reduces the oxide back to the metal freeing the oxygen to further corrode and remove more of the metal from the filament or electrode and further clouding the envelope by depositing the metal on the interior surface thereof. Thus, the lamp envelope becomes increasingly cloudy with a corresponding reduction in the efficiency of the lamp, and the metal filaments are deteriorated reducing the useful life of the lamps.